Documentation Improvements and PEP8 Compliance

PEP8 Compliance
---
PyPXE is now 100% PEP8-compliant!
* All files were converted to space-based indents rather than tab-based indents with 4 spaces per indent
* All whitespace recommendations and best-practices are now implemented everywhere
* All imports are listed individually rather than importing some of them on one line
* All function parameters are indented properly (only if they're ridiculously long)

**NOTE:** No actual code was changed, just updates to conform to PEP8 style guidelines

Documentation Improvements
---
Reorganized and reformatted ```README.md``` and ```DOCUMENTATION.md``` so that everything is more easily understandable and to the point.

```README.md``` now has basic information and comprehension of what PyPXE is and how to get started. The ```server.py``` arguments are more easily readible and explained.

```DOCUMENTATION.md``` is referred to in several places in ```README.md``` so that users know it exists and is also formatted better for readability. I started writing the section on the TFTP service (not finished, but started it).

Minor grammatical fixes and improvements throughout all documentation were made.