Made the JavaScript more standard by avoiding `substr`

I've made the JavaScript more standard by avoiding the substr string method, which is non-standard.

Fixed a long-standing bug in the parser regarding unary operators

I've fixed a bug regarding the detection of unary operators. Previously, the parser would detect the second + in 5 + + 1 as a binary operator, leading to a parsing error.

Fixed a bug in `headerScript.js` where choosing an example program does not count as modifying the code after assembly

I've fixed the #29 . As well, I've made some documentation fixes.

Fixed a bug in the UI

I've fixed the bug #28 .

Added a modal dialog warning the user if they forgot to click the Assemble button

I've added a modal dialog confirm warning in case the user has forgotten to press the Assemble button before pressing Play or Fast Forward. Also, I've done some minor refactoring.

Used CSS Media Queries instead of JavaScript where that is easy to do

So, I'm taking a Front-End Development course at Algebra and we were taught that we should use CSS Media Queries where possible instead of JavaScript. So, I've done that.

Fixed a bug causing the assembler to fail silently in case of an invalid decimal or binary constant

I've fixed the bug #27 .

Restored compatibility with WebPositive

The names of some example programs weren't legible in WebPositive, so I fixed that using browser-sniffing in footerScript.js.

Improved what the long tables look like

I have changed the CSS-in-JavaScript of the long tables, to make the borders of the sticky <th>-s shown in Firefox (see this bug in Firefox).

Made the headers of the long tables sticky

I have made the headers of the long tables (one with machine code and one with the memory) sticky, so that users don't lose the track of what is in which column.