Support for 'npm' package.

Exports declared.
package.json added.
Bump version number. Updated package.json. (#18)

package.json:
- Using consistent version numbers
- Pointing at source repo after cherry-picking from @dominictarr's fork.
- Removed duplicate 'main' definition
- Extended keywords
Consistent code formatting: indentation and spacing.
Calculate threshold time value once per render, not N.
Renamed 'canvasContext' to 'context' for brevity.
Extracted calculation of min/max value range to function.

Rearranged some aspects of the render function for clarity.
Extracted function for dropping old data.

Improved performance when dropping multiple points by only calling splice once (an O(N) operation itself), rather than N times.
Using === for performance. Moved comment.
Updated changelog.
Fixed bug in SmoothieChart.removeTimeSeries where the *last* TimeSeries was always being removed, not necessarily the provided one. Closes #24. Thanks to @davidgaleano for providing the fix!