Update README and add PyPI badge [skip ci]

[mbrukman]

Global changes:

• use leading # to specify heading level rather than underlines; it's easier
  to manage and does not require an extra line or keeping the underline matching
  the length of the heading

Initial section:

• update title to be "ipycache" instead of description, since it's already in
  the first line of the README
• add badge displaying the latest PyPI version and linking to PyPI

Installation section:

• put installation commands into separate block commands rather than inline code

Examples section:

• link to all 3 notebooks in the examples directory
• link to notebooks using relative links, since GitHub can now render notebooks

Usage section:

• remove bullet points in "Usage" section, since every paragraph was its own
  bullet, and it's not really a list anyway, so it's less readable this way
• replace tabs with spaces for indentation of code

[mbrukman]

@rossant — I know you said I should feel free to merge my own PRs, but since I'm changing what is the public image of this project's repo (the README), including changing the title, I figured you might want to take a look at this before I merge it, since it goes live immediately.