Don't write None to local cache
Set rate in cache after reading from db
Update README.rst

my_price does not have a convert method. Also converting to 'YEN' did not work for me.
Update README.rst

updated output
Merge pull request #23 from tatterdemalion/master

Updated Documentation
Fixed bug preventing updated rates being saved to db
Bumped version to 0.5.2
Merge remote-tracking branch 'metglobal/master'

Conflicts:
	exchange/__init__.py
Merge pull request #24 from fergalwalsh/master

Fix bug in adapter.update and bump to 0.7.2