Adding default values for event parameters

[beasteers]

Here is a summary of the defaults that I added:

# Background:
label: ('choose', [])
source_file: ('choose', [])
source_time: ('uniform', 0, source_duration - duration)
snr: ('const', 0)

# Foreground
label: ('choose', [])
source_file: ('choose', [])
source_time: ('const', 0)
event_time: ('uniform', 0, duration - event_duration)
event_duration: ('const', source_duration)
snr: ('const', 0)
pitch_shift: None
time_stretch: None

The ones that are most beneficial are the source_time, event_time, and the event_duration, as they can help sample evenly from recordings with differing lengths with fewer manually set values, but defaults in general seem like a nice convenience.

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This change is 

[beasteers]

This is in reference to #41

[coveralls]

Coverage remained the same at 100.0% when pulling 90e2f00 on bensteers:event_defaults into 3c83bba on justinsalamon:master.

[pseeth]

This looks somewhat related to the changes in #53, if you want to take a look at that. Instead of hard-coding defaults for source_time, you can pass in something like ('uniform', 0, 100000), where the last number is just some number bigger than the duration of all your source files and it'll fall back to doing ('uniform', 0, source_duration - event_duration). Similar logic is implemented for the other supported distributions.

[beasteers]

Oh wow that's some timing lol. K, I'll look into it when I can get a minute. Hopefully we can merge them, because I still think having default values across the board is intuitive, like I think you should be able to do:

sc.add_background(label=('const', 'street'))
sc.add_event(label=('const', 'bird'))
sc.add_event(label=('const', 'car_honking'))
sc.add_event() # any random event

and have it generate valid soundscapes based on the limited information you give it.

[justinsalamon]

See my latest comment on #41