Setuptools-scm

- fix regression caused by the fix of 101
- assert types for version dumping
- strictly pass all versions through parsed version metadata

- fix issue 97 - add support for mercurial plugins
- fix issue 101 - write version cache even for pretend version
(thanks anarcat for reporting and fixing)

- fix issue 88 - better docs for sphinx usage (thanks Jason)
- fix issue 89 - use normpath to deal with windows (thanks Te-jé
Rodgers for reporting and fixing)

- always run tag_to_version so in order to handle prefixes on old
setuptools (thanks to Brian May)
- drop support for python 3.2
- extend the error message on missing scm metadata (thanks Markus
Unterwaditzer)
- fix bug when using callable version_scheme (thanks Esben Haabendal)

- fix issue 73 - in hg pre commit merge, consider parent1 instead of
failing

- add support for overriding the version number via the environment
variable SETUPTOOLS_SCM_PRETEND_VERSION

- fix issue 63 by adding the `--match` parameter to the git describe
call and prepare the possibility of passing more options to scm
backends

- fix issue 70 and 71 by introducing the parse keyword to specify
custom scm parsing, it's an expert feature, use with caution

this change also introduces the setuptools_scm.parse_scm_fallback
entrypoint which can be used to register custom archive fallbacks