Version 0.4.0

Changed a submission flow - from spawning and sending data from a process to a background thread(#19).
Added support for submit.backtrace.io(#14).
Added new default attributes:
- backtrace.agent, backtrace.version (#13)
- application.session (#13),
- uname.sysname, uname.version, uname.release (#13),
- error.type (#13),
- linux process and system memory information (#15),
- guid (#15),
- cpu attributes (#15),
- process.age (#15),
Added a support for attachments (#19),
Added a new client initialization attributes:
- attributes - client specific attributes that will be included every time the report is being generated (#19),
- attachments - list of attachments paths (#19),
- collect_source_code - disable collecting source code information for each stack trace (#20),
- ignore_ssl_certificate - if True, the option disables ssl validation (#19).

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