Copy testInstallation.py to the simtk package and make it an import-safe module.
Also copy the test PDB file to the data/ directory, since we rather need it...
Delete all of the code from testInstallation.py and instead replace it with an
import of the new testInstallation module and a command to run the tests.

This way, we only have one place where we need to maintain this code, yet the
current workflow of "run testInstallation.py" still works.
Apparently "from XXX import *" is only allowed at the module level (I get a
SyntaxWarning otherwise). So move those imports back to the module level and
save any error messages for when the tests are actually run (if ever).

python -m simtk.testInstallation

runs the tests, whereas

python -c "import simtk.testInstallation"

does not.
Tweak error message formatting.
Further formatting tweaks.
Add docstring to test function. Convert all raised exceptions raised in
run_tests to TestingError so users only ever have one exception to catch.
Add "python -m simtk.testInstallation" to .travis.yml
Merge pull request #739 from swails/conda-testing

Conda testing
Updated instructions on running the test installation script
Merge pull request #740 from peastman/test

Updated instructions on running the test installation script