Support alternate PLY property type names

This commit adds support for the int8, uint8, etc. property type names
in addition to the char, uchar, etc. that were supported before.
add contribution guidelines
Update CONTRIBUTING.md
We can set the Scale Factor in LAS/LAZ files.

PotreeConverter takes a new parameter --scale (default 0.01).
Merge pull request #73 from dranjan/bugfix-ply-support

Bugfix ply support
Merge pull request #72 from Antea/scale

We can set the Scale Factor in LAS/LAZ files.
make 0.01 default scale; prepare for making .bin the default extension
New parameter auto-spacing that calculates spacing on the boundingbox diagonal.
fixed bounding box precision
Merge pull request #76 from Antea/auto-spacing

New parameter: auto-spacing (see #17)
added changelog; refactoring
Update changelog.md
Update README.md
Merge pull request #77 from potree/develop

Develop