Modify formatting and grammar

+ Modify line-length to 80 columns.
+ Remove unnecessary blank lines.
+ Replace tabs for spaces.
+ Replace `**` used as bold character for `__` to be consistent with the italics character `_`
+ Replace `-` list character for `+` to avoid confusing it with bold or italics characters.
+ Replace long GitHub URIs by short ones.
+ Fix some typos and misspellings.