Release 0.2.0

What's new?

Features:

• Implement arithmetic methods for List type - add, sub, mul, div, add_scala, sub_scala, mul_scala, div_scala and pow_scala;
• Implement data processing methods for List type - copy, filter, sort, to_list and unique;
• Implement a new type BooleanList;

Misc:

• Add a docker file to distribute ulist for Linux;
• Add develop/build/publish/docker instructions in README.md.
• Put the List trait and Boolean type in a new file from current lib.rs file.