BLD: setup.py version strings. handle shallow clones and installing from sdist

[ghost]

Travis exposed a corner case in the handling of version strings since the git-describe shakeup.

Previously (0.10.1), version strings were of the form:

v0.10.1 # release
v0.10.1.dev # dev, no git available
v0.10.1.dev-abcd1234 # dev, git available

Now that we use git-describe, here's the case breakdown:

• release
• dev, no git
• dev, no git and pandas/version.py exists (installing from sdist) # *
• dev, git available
• dev, git but shallow clone (no tags) (travis) # *

the * cases were hard to see coming (I didn't) and so the current logic does not handle
them gracefully, and weird version strings result.

Behavior summary following this PR:

v0.13.1 # release
v0.13.1.dev # dev, no git available, matches old behavior
v0.13.1.dev-abcd1234 # dev, git available but no tags (pip tarball), matches old behavior
foo # dev, no git available, pandas/version.py exists and contains foo
0.13.0-509-ga134eb6 # dev, git available, tags available (normal dev situation), normal new behavior.

Slightly dangerous to merge shortly before a release, but it would be worse
to keep this brokenness for months until 0.14 IMO.

[ghost]

On second thought, only affects dev version strings anyway, pushing to 0.14.