Various spelling/grammar/punctuation fixes, and bugfix for unicode (or lack thereof) in ST3

I'm a bit pedantic when it comes to spelling and stuff like that, so I originally forked the repo just to clean things up a little bit. However, I ran into an error using ST3:

```python
File "MarkdownPreview in C:\XXX\Sublime Text 3\Data\Installed Packages\Markdown Preview.sublime-package", line 199, in get_javascript
NameError: global name 'unicode' is not defined
```
The fix turned out to be pretty simple: just change this (in `MarkdownCompiler.get_javascript()`):
```python
if js_files is not None:
    # Ensure string values become a list.
    if isinstance(js_files, str) or isinstance(js_files, unicode):
        js_files = [js_files]
```
to this:
```python
if js_files is not None:
    # Ensure string values become a list.
    if sublime.version() >= '3000':
        if isinstance(js_files, str): # don't have unicode in Py3
            js_files = [js_files]
    else: #2.x
        if isinstance(js_files, str) or isinstance(js_files, unicode):
            js_files = [js_files]
```

For some reason the pull request is showing several other changes to the file, it may just be trailing spaces removed, etc.

Anyways, I hope all this is helpful. I just started using this plugin today, and I like to contribute where I can, so if I come across anything else I'll let you know!

Matt