Extract ParseSize from juju/instance

The ParseSize function will be used to parse
the size in storage specifications.
Revert "Revert "SetFile -> SetFileInfo.""

This reverts commit fe902901d130b8f780f4f7c1e0127e15968e9a57.

It effectively re-merges e7b7868dea3aad14476bc95245529cd5f39b790b.
Merge pull request #88 from ericsnowcurrently/revert-revert-SetFileInfo

Revert "Revert "SetFile -> SetFileInfo.""
Drop Copy() from the Document interface.
Merge pull request #89 from ericsnowcurrently/filestorage-drop-Metadata-Copy

Drop Copy() from the Document interface.
Merge pull request #82 from axw/utils-parsesize

Extract ParseSize from juju/instance
Fixed lp:1394524 - apt-get retry should not call CombinedOutput twice
Merge pull request #91 from dimitern/lp-1394524-fix-apt-retry

Fixed lp:1394524 - apt-get retry should not call CombinedOutput twice
Created MakeFileURL function that creates a proper file:// URL for both
platforms