Import basic modules into __init__.py

This patch allows users to use `import laspy` to access the basic
laspy classes and functions, e.g. `laspy.file.File`.
Merge pull request #5 from gadomski/master

Import base, file, etc into __init__.py
Add documentation for absolute imports

This documentation consists of a short note at the top of the first
tutorial page, indicating that while the documentation uses relative
imports in code examples, the reader can use absolute imports if they
choose. The documentation also includes a few topical links.
Fix some spelling mistakes
Merge pull request #7 from gadomski/absolute-import-doc

Add documentation for absolute imports
Merge pull request #8 from gadomski/spelling-fixes

Fix some spelling mistakes