Upgrade filter and match to use shared_ptr to connection instead of raw reference

Basic dbus cleanup

merging changes from @cawka to close #20

Added explicit asynchronous start function

A `connection` can operate synchronously, which
involves less overhead. It will be in a synchronous-
only state when it has been newly cons

Added explicit asynchronous start function

A `connection` can operate synchronously, which
involves less overhead. It will be in a synchronous-
only state when it has been newly constructed or cancelled.

`dbus::impl::connection::start()` begins/resumes
asynchronous operation, and should be called automatically
whenever an asynchronous operation is requested.

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moved match/filter implementation details to dbus::impl

added license file and copyright comments - fixes #10

Tweaks to message, filter and error

- `message` now has get_path() etc. for accessing its metadata.
- `error` how has a constructor which takes a `message`,
to ease conversion of e

Tweaks to message, filter and error

- `message` now has get_path() etc. for accessing its metadata.
- `error` how has a constructor which takes a `message`,
to ease conversion of error messages to throwable exceptions
or `error_code`s
- `filter`'s predicate now takes `message`s by reference and
is not ignored

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filter, match brought to sanity

match/filter hackathon

Mostly syntactically correct test.cpp

connection_service sketch