Fixes for Python API

[thowell]

• Python module looks in build directory for task models
• Change filepath for quadruped task_flat to build directory to fix Python test
• Minor update to README mentioning to first build MJPC
• Add .eggs to gitignore
• Change filepath to build for agent_test.py since #256

The change to the build directory enables us to utilize "patches" (#256) for task models without rewriting the python/setup.py.

[thowell]

@nimrod-gileadi is this the correct way to check that the build exists and provide the user with an informative error if it is missing?