Add repository search

Qualifiers can be added via keyword args in order to keep the method
uncluttered. Qualifiers could also be given in the query string
directly.
Add user search

Qualifiers can be added via keyword args in order to keep the method
uncluttered. Qualifiers could also be given in the query string
directly.
Add issue search

Qualifiers can be added via keyword args in order to keep the method
uncluttered. Qualifiers could also be given in the query string
directly.
Add code search

Code search results are content resources with an embedded repository.
The "content" and "encoding" attributes aren't returned and require
completion.
Add decoded_content property to ContentFile

The only encoding currently supported is base64, but that's ok, since
v3 of the GitHub API only mentions base64.
Fix add_attribute insertion logic

The code didn't work if it couldn't find a property in the file
that is lexicographically greater than the new attribute.
It now accepts _identity as an "end point" to insert before, and
empty lines in _initAttributes() and _useAttributes(), as they mark
the end of the method.

Also add "float" as a possible type, mainly for the "score" attribute
in search results.