Make scripts runnable in macOS and Linux

[yam655]

Scripts should now be runnable with ./script.py. This uses #!/usr/bin/env python3 so python3 just needs to be on the path. (This lets it work both in Linux as well as in macOS when using brew or other third-party package manager.)

Added a few lines to .gitignore:

• venv -- for venv support
• .eggs/ -- non-Windows eggs/ directory.
• asciimatics/version.py -- file started out with a statement to not put it in version control, so it seemed an ommision.
• *.log -- At least one sample, when run, created a .log file. Logs are not normal things to check in.

I had these files created, but they were not checked in, and it seemed like they should never be checked in.

Issues fixed by this PR

Closes #248

What does this implement/fix?

This makes the sample scripts more easily runnable on the command-line.

Any other comments?

I love the product!

[coveralls]

Coverage remained the same at 94.615% when pulling 5873e0284bd0aad932458e6c68e18c7e8c5cb609 on yam655:feature/runnable-samples into 676097d12150bc09e6b5a7ca6c471b7f97c1b9f4 on peterbrittain:master.

[peterbrittain]

Thanks! All looks good to me.