added xmatch docs
removed blank lines
formatting fixes
trivial docs edit
Merge branch 'docs_xmatch' of github.com:keflavich/astroquery into keflavich-docs_xmatch

Conflicts:
	docs/xmatch/xmatch.rst
Merge branch 'keflavich-docs_xmatch' into docs/xmatch
Merge pull request #397 from derdon/docs/xmatch

Added xmatch documentation